BASE(): support two's complement negative binary numbers

hi,
I saw here:
http://docs.oasis-open.org/office/v1.2/cd05/OpenDocument-v1.2-cd05-part2.
html#a_6_19_3_BASE
that BASE() require 'Integer X' >=0.

This forbid conversion of negative numbers to binary. The problem is ever 
whorse when the number is lesser than -512,
as DEC2BIN and HEX2BIN require two's complement with a max of 10 digit.

So please add support for two's complement negative binary numbers as output 
of BASE().

thank you in advance,
Valerio